The ultrasound diagnostic apparatus transmits ultrasound pulses from an ultrasound probe to the inside of a living body, receives ultrasound echoes scattered or reflected within the body via the ultrasound probe, and applies various signal processes to thus received ultrasound echoes (received echoes), thereby obtaining a ultrasound B-mode image and a blood flow image, and this apparatus is widely used for medical diagnosis.
Waveforms of the ultrasound waves applied to the living body are distorted along with propagation. This is because acoustic waveforms have acoustic nonlinearity, that is, the waveforms proceed rapidly in a portion with high sound pressure, whereas it proceeds slowly in a portion with low sound pressure. This waveform distortion accumulates along with the propagation of acoustic waves. Occurrence of the waveform distortion indicates occurrence of a higher harmonic component or a low-frequency harmonic component assuming the transmitted acoustic wave as a fundamental wave component, in other words, indicating occurrence of a nonlinear component. This nonlinear component occurs in a broadband, in proportion to approximately the square of sound pressure amplitude. Therefore, by creating an image from the nonlinear component, it is possible to obtain an image that excels in contrast resolution and spatial resolution. This type of imaging method is generally referred to as THI (tissue harmonic imaging).
As one method of imaging by the ultrasound diagnostic apparatus, there is an ultrasound contrast imaging method that uses an ultrasound contrast agent. The ultrasound contrast imaging method intravenously injects into a living body, a preparation obtained by stabilizing micro bubbles in micron order size as the ultrasound contrast agent, and then performs ultrasound imaging. This method is widely used for diagnosing disease that is reflected on blood vascular system, such as malignant tumor and infarction. This micro-bubble type ultrasound contrast agent shows an extremely strong nonlinear response to a few MHz ultrasound wave that is mainly used in ultrasound diagnosis. Therefore, the nonlinear component of the ultrasound echoes in the ultrasound contrast imaging method includes a large amount of ultrasound echoes coming from the ultrasound contrast agent. An imaging method that extracts such ultrasound echoes in the nonlinear component and creates an image therefrom, so as to visualize a vascular structure, and the like, is generally referred to as CHI (contrast harmonic imaging).
As described above, in the THI or the CHI (if it is not necessary to make a distinction therebetween, they are collectively referred to as “harmonic imaging”), an image is created by using the acoustic nonlinear characteristics of acoustic wave propagation through a living body, or the nonlinear component generated on the basis of the nonlinear characteristics in the oscillation of contrast agent. In the ultrasound echo, there exist a fundamental wave component included originally in the transmitted acoustic wave and the aforementioned nonlinear component in a mixed manner, and therefore, it is necessary to extract the nonlinear components from the received echoes. As the method for extracting the nonlinear component from the ultrasound echoes, there are a method for separating the nonlinear component by using a filter (e.g., see the patent document 1), PI (pulse inversion) method (e.g., see the patent document 2), and an amplitude modulation method (e.g., see patent document 3).
The PI method transmits two ultrasound pulses respectively having acoustic wave pulses being inverse, positive and negative, to an identical portion of the living body, and sums the reflection echoes therefrom. Since a fundamental wave component behaves linearly, when the transmit pulses being inverse each other are transmitted, the fundamental wave components of the reflection echoes are also inverse each other, and they cancel each other out when they are added together. On the other hand, the nonlinear components are distorted differently depending on whether the sound pressure is positive or negative. Therefore, even though the transmit pulses being inverse each other are transmitted, they do not form waveforms being inverse and they do not cancel each other out when they are added together. Eventually, when the reflection echoes of the transmit pulses being inverse each other are added together, only the nonlinear components remain.
As described in the patent document 3, the amplitude modulation method performs transmit of ultrasound waves twice, similar to the PI method, and as for the pulse in the second transmit, its acoustic waveform is not inverted, but sound pressure level (amplitude) is made lower than the pulse in the first transmit. By way of example, in the second transmit, the sound pressure amplitude of the pulse is made half of the first transmit pulse. Then, the reflection echo of the second transmit pulse is doubled and subtracted from the reflection echo of the first transmit pulse, thereby removing the fundamental wave components within the reflection echoes. When the amplitude modulation method is applied to the THI, the fundamental wave components are canceled out, and only the nonlinear components remain. When the amplitude modulation method is applied to the CHI, it is possible to extract not only the higher harmonic component of the contrast agent origin, but also the nonlinear component dependent on the sound pressure amplitude of the contrast agent origin, enabling an ultrasound contrast image with a high CTR (contrast-to-tissue ratio) to be obtained.
In addition, a harmonic imaging combining the amplitude modulation method and the PI method is also being devised.
On the other hand, in the imaging method using the PI method or the amplitude modulation method, it is necessary to turn the phase of the transmit voltage waveform by 180 degrees so as to invert the sound pressure waveform, or vary the sound pressure amplitude while maintaining the sound pressure waveform. Therefore, if there is any distortion dependent on the voltage amplitude and phase and/or nonlinear characteristics in the transmit system of the ultrasound diagnostic apparatus that incorporates a transmit amplifier, an ultrasound probe, and the like, it is not possible to remove the fundamental wave component sufficiently.
In order to solve this problem, there is a measure to perform the amplitude modulation method in a transmit sound field by synthesizing transmit aperture (e.g., see the patent document 4). An ultrasound probe is provided with channels made up of plural ultrasound transducers. Those channels are assigned in such a manner that plural channels for transmitting a first transmit pulse P1 and plural channels for transmitting a second transmit pulse P2 become mutually different, entirely or partially. Furthermore, the channel for transmitting a third transmit pulse P3 uses both the channel used for transmitting the first transmit pulse P1 and the channel used for transmitting the second transmit pulse P2. With this configuration, the transmit sound field of the third transmit pulse P3 is obtained by linearly combining the transmit sound field of the first transmit pulse P1 and the transmit sound field of the second transmit pulse P2. Therefore, the operation of P3−(P1+P2) may remove a linear fundamental wave component and extracts the nonlinear component.